Effect of Cation Structure of Ionic Liquids on Anode Properties of Si Electrodes for LIB

Abstract

Ionic liquids consisted of 1-((2-methoxyethoxy)methyl)-1-methylpiperidinium (PP1MEM) or 1-hexyl-1-methylpiperidinium (PP16) and bis(trifluoromethanesulfonyl)amide (TFSA) were applied to an electrolyte for Li-ion battery. The effect of their cation structure on anode properties of Si electrodes were investigated through the use of thick film prepared by gas-deposition without any binder and conductive additive. The Si electrode in PP1MEM-TFSA exhibited an initial reversible capacity of 2670 mA h g −1 , which is larger than that in PP16-TFSA by ca. 900 mA h g −1 . Moreover, a comparatively high capacity of 1150 mA h g −1 at a high current density of 4200 mA g −1 is achieved in PP1MEM-TFSA whereas the Si electrode in PP16-TFSA showed the capacity of only 210 mA h g −1 . Raman analysis and electrochemical impedance measurements revealed that PP1MEM cation played a role reducing the interaction between Li ion and TFSA anions, and that Li-ion transfer at the electrode−electrolyte interface in PP1MEM-TFSA was remarkably improved compared with PP16-TFSA. These results indicate that the excellent performances obtained in PP1MEM-TFSA originate from a smooth Li-insertion into Si electrode. It was suggested that introduction of ether functional group into cation is valid to enhance the electrode performance.Li-ion battery (LIB) has been utilized in large-scale battery systems such as power supply of electric vehicles in recent years. Unfortunately, the conventional LIB using graphite anode has insufficient energy density to satisfy the growing demand. Silicon is one of promising candidates of anode materials for next-generation LIB due to its huge theoretical capacity compared with that of graphite practically used. Si and Li form several Li-rich binary alloy phases such as Li 15 Si 4 at room temperature and Li 22 Si 5 at a high temperature of 415 • C, which offers extremely high theoretical capacities of 3580 and 4200 mA h g −1 , respectively. 1-3 The volume of Si is, however, significantly changed during alloying/dealloying reactions. The volumetric change ratios per Si atom from Si to Li 15 Si 4 and Li 22 Si 5 correspond to 380% and 410%, which results in a generation of high stress and large strain in the active material. The strain accumulated by repeating charge−discharge cycle causes a disintegration of Si electrodes, leading to a rapid capacity fading and a poor cycle stability. In addition, Si has disadvantages of a low electrical conductivity and a low diffusion coefficient of Li + in it (D Li+ , 10 −14 to 10 −12 cm 2 s −1 ). 4-6 For these reasons, a practical application of Si electrodes has been hindered. To overcome these problems, considerable attempts have been carried out. Many researchers have studied composite materials consisted of carbon, with mechanically soft property and good electrical conductivity, and pure Si. 7-10 We have recently demonstrated that composite electrodes prepared by using LaSi 2 /Si and Ni−P/Si exhibit a high anode performance because LaSi 2 and Ni−P effectively compensate for silico...

Elucidation of the Reaction Behavior of Silicon Negative Electrodes in a Bis(fluorosulfonyl)amide‐Based Ionic Liquid Electrolyte

“…In contrast, in the PP1MEM‐FSA, the electrode showed not only a high initial discharge capacity of 2700 mA h g −1 but also excellent cycling stability: the discharge capacity was approximately 950 mA h g −1 even at the 500th cycle. We previously reported that a Si electrode exhibited a discharge capacity of 1000 mA h g −1 at the 100th cycle in 1 M LiTFSA/PP1MEM‐TFSA ,. Although these ionic liquids consisted of the same PP1MEM cation, the PP1MEM‐FSA achieved superior cycling performance.…”

“…Thus, this method is suitable for elucidating the reaction behavior of a Si electrode. The detailed conditions have been described previously ,. The weight of the deposited active materials, the deposition area on the Cu substrate, and the thickness of the active material layer were 30±2 μg, 0.5×0.5×π cm 2 , and ca.…”

“…The ionic liquid electrolyte solution was 1 mol dm –3 (M) lithium bis(fluorosulfonyl)amide (LiFSA) dissolved in 1‐((2‐methoxyethoxy)methyl)‐1‐methylpiperidinium bis(fluorosulfonyl)amide (PP1MEM‐FSA). PP1MEM cation was used because it can increase the initial capacity of the Si electrode . For comparison, 1 M lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) in propylene carbonate (PC, Kishida Chemical Co., Ltd.) was also used as a conventional organic electrolyte.…”

“…Some researchers reported negative‐electrode performance of electrodeposited Si electrode in lithium bis(trifluoromethanesulfonyl)amide/1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethanesulfonyl)amide, which was relatively high cycling stability compared to that in an organic electrolyte . We previously investigated the electrochemical performance of Si‐based electrodes in various ionic liquid electrolytes . We found that the cycling stabilities of Si‐alone electrodes were significantly improved in some ionic liquid electrolytes.…”

Applicability of an Ionic Liquid Electrolyte to a Phosphorus‐Doped Silicon Negative Electrode for Lithium‐Ion Batteries

“…Ionic liquids have received much attention as alternatives to flammable organic electrolytes consisting of carbonate‐based solvents because of their physicochemical properties, wide electrochemical windows, and high thermal stability. We have investigated the applicability of ionic liquid electrolytes to Si‐based electrodes . As a result, the cells showed not only high safety but also excellent electrochemical performance in certain ionic liquid electrolytes.…”